One of the pathologic hallmarks of Alzheimer's disease is a profound loss of cholinergic neurons in the forebrain. This neuronal loss has been strongly associated with the memory impairment and other cognitive deficits found in ad patients. In addition, positron emission tomography (pet) studies have identified a characteristic, specific pattern of cerebral metabolic changes in ad: bi-lateral reductions in glucose metabolism in parietotemporal areas, beginning early in the disease process, with reductions in frontal areas occuring later on.